During development of the central nervous system (CNS), oligodendrocyte progenitor cells are generated in discrete germinal zones from where they migrate to prospective myelinated areas. At these final destinations, oligodendrocytes become premyelinating, post-migratory cells that extend a large and complex process network in search for axonal segments to be myelinated. Upon appropriate target interaction, premyelinating, post-migratory oligodendrocyte processes transition from process outgrowth to membrane sheet formation. These critical final and distinct steps of oligodendrocyte maturation are regulated by complex interactions of the oligodendrocyte's protrusions, i.e. processes, with their extracellular environment, and they are considered crucial for efficient myelination. However, little is known about the extracellular factors and molecular mechanisms coordinating the intricate sequence of events that occurs during the progression from a few process bearing to a complex process network extending and then to a myelinating cell. Thus, as an approach toward a better understanding of the regulation of myelination, the long-term goal of these studies is to gain insight into the regulation of outgrowth and maturation of protrusions extended by premyelinating, post-migratory oligodendrocytes. Data generated in our laboratory suggest that the matricellular protein phosphodiesterase-I1/autotaxin (PD-Ia/ATX), which is released by premyelinating, post- migratory oligodendrocytes during the initial stages of myelination, plays a crucial role for the maturation of oligodendrocyte protrusions. Based on our published and preliminary data we formulate the central hypothesis that PD-Ia/ATX promotes the progression from an early stage premyelinating, post- migratory oligodendrocyte into a fully functional, i.e. myelinating cell, via the concerted action of three distinct functionally active sites. In particular, we are planning to complete the following studies to address the above stated central hypothesis: 1) We will characterize in vitro the molecular mechanisms that are mediated by the proposed two functionally active sites of PD-Ia/ATX's MORFO domain and that are involved in promoting the maturation of premyelinating, post-migratory oligodendrocyte protrusions into a complex process network. 2) We will characterize in vitro the molecular mechanisms that are mediated by PD-Ia/ATX's lysoPLD active site and that are involved in promoting the maturation of premyelinating, post-migratory oligodendrocyte protrusions into membrane sheets. 3) We will characterize in vivo the role of PD-Ia/ATX and its functionally active sites on the remodeling of oligodendrocyte protrusions and on initial myelination using the zebrafish as a model system. The data obtained in these and subsequent investigations are likely to significantly advance not only our knowledge about developmental myelination but they may also unravel novel therapeutic targets for improving remyelination. PUBLIC HEALTH RELEVANCE: Multiple Sclerosis (MS) is the major demyelinating disease in human and after neurotrauma the most common disabilitating neurological disease in young adults. Unfortunately endogenous repair of the myelin sheath is limited within the CNS and currently no remyelination-promoting therapies exist, despite the fact that remyelination is currently the best proven strategy for functional recovery and neuroprotection. As an attempt to better understand the regulatory circuits that may be necessary for stimulating remyelination, the present grant application investigates the role of the extracellular factor phosphodiesterase-Ia/autotaxin (PD-Ia/ATX) in promoting the maturation of the cells responsible for generating myelin, namely premyelinating, post-migratory oligodendrocytes.